1. Field of the Invention
The present invention relates to high voltage d.c. supplies in which it is desired to regulate the output voltage to avoid the effects of supply line or load variations. The invention also relates to supplies in which active regulation is used to eliminate a ripple.
2. Description of the Prior Art
Present high voltage generators for X-ray applications typically use an "open loop" primary a.c. voltage control to establish the magnitude of the generated high voltage. In a similar open loop manner, the X-ray emission quantity is established by pre-setting the filament power applied to the X-ray tube filaments. The X-ray tube is operated in an emission limited mode. The duration of the X-ray exposure is controlled via neutral switches on the primary of a three-phase high voltage transformer.
This approach is sometimes augmented by using a triode X-ray tube to gate the anode current and thus the X-ray emission. The triode X-ray tubes are much more expensive and less reliable than the standard diode X-ray tubes. Another approach that has been used, is to insert high voltage tetrode vacuum tubes in series with the high voltage connections between the high voltage generator and the X-ray tube. The purpose of these approaches is to provide a faster control for the X-ray emission than can be provided via the primary neutral switching scheme.
The open loop control of the high voltage generation requires a substantial amount of additional equipment for a.c. voltage regulation. For example, a tapped three phase line transformer is supplied to pre-condition the line voltage. The final adjustment of the generated high voltage is achieved by means of a motor driven three phase variac that sets the primary a.c. voltage on the high voltage transformer.
In addition, the design of the high voltage transformer in conventional X-ray apparatus requires a roughly cubic or cylindrical shape (of significant bulk) for efficient utilization of core material and minimization of core losses. The bulk and shape factor has tended to restrict the mobility of the apparatus, and flexibility in mechanical design.